1. Field of the Invention
The present invention relates to a crystal oscillator circuit having a variable-capacitance element whose capacitance value can be controlled by a voltage to adjust the oscillator output frequency.
2. Description of the Related Art
Crystal oscillator circuits utilizing crystals offer enormous advantages in practical applications, since the crystals exhibit extremely high frequency stability, have an excellent temperature characteristic, and are easy to manufacture. Because of these advantages, crystal oscillators constructed as single packages by incorporating crystals into oscillator circuits, and capable of producing a desired-clock frequency by just applying a prescribed voltage, have been used widely in recent years.
In the field of electrical communications, etc., to maintain synchronization between a plurality of signals, or to synchronize a system clock to a transmission carrier wave, the frequency of the crystal oscillator must be made tunable within a certain range. To tune the oscillation frequency of the crystal oscillator, it is commonly practiced to use a variable-capacitance element as a load capacitor for the crystal. A varicap, and the like, with a capacitance which can vary with an applied DC voltage is used as the variable-capacitance element. A crystal oscillator having such a function is specifically called a voltage-controlled crystal oscillator.
FIG. 1(a) shows one configuration example of a voltage-controlled crystal oscillator circuit according to the prior art, and FIG. 1(b) shows a two-terminal equivalent circuit of a crystal. In FIG. 1(a), reference numeral 1 is the crystal, 2 is an inverting amplifier, 3 is an input terminal of the oscillator circuit, 4 is an output terminal of the oscillator circuit, 5 is a buffer amplifier, 6 and 7 are control voltage application terminals, R is a resistor, Cv is a variable-capacitance element, Ccut is a DC cut capacitor element and Cp1 and Cp2 are possible parasitic capacitors. For actual operation of the circuit, a suitable bias voltage must be applied to the input terminal 3 of the oscillator circuit. In the figure, it is assumed that the bias voltage application means is included in the inverting amplifier 2. In FIG. 1(b), C0, L1, C1, and R1 are equivalent circuit constants for the crystal: C0 represents an equivalent parallel capacitance, L1 an equivalent series inductance, C1 an equivalent series capacitance, and R1 an equivalent series resistance.
FIG. 2 is a cross-sectional view showing an integrated load capacitor portion (at one side) when the voltage-controlled crystal oscillator circuit shown in FIG. 1(a) is implemented in integrated circuit form. In FIG. 2, reference numeral 10 indicates a silicon substrate and 11-14 represents a varicap, i e., the variable-capacitance element, integrated on the substrate. Here, reference numeral 11 indicates a lightly doped diffusion layer called the p-well, 12 a highly doped diffusion layer called the p+ region, 13 a highly doped diffusion layer called the n+ region, and 14 a depletion layer. The highly doped diffusion layer 12 is at ground potential. Further, reference numeral 15 designates a field oxide film formed on the surface of the substrate, and 16-18 represents the DC cut capacitor element, having two polysilicon layers, formed on top of the field oxide film 15. Here, reference numerals 16 and 17 are polysilicon films as electrode films, and 18 is an insulating layer. Further, reference numeral 19 indicates a metal wiring line which connects the DC cut capacitor element in series with the variable-capacitance element and also connects to the resistor R (FIG. 1(a)). The control voltage is applied to the variable-capacitance element 11-14 via the resistor R (FIG. 1(a)) and via the metal wiring line 19. The depth of the depletion layer 14 varies with the magnitude of this control voltage, causing the capacitance value (the capacitance between the n+ region and the p+ region) of the variable-capacitance element to change. The upper electrode 17 of the DC cut capacitor element is connected to the input terminal 3 (FIG. 1(a)) or the output terminal 4 (FIG. 1(a)) via a metal wiring line (not shown). Dashed line 20 indicates a metal wiring line for the reverse connection described later in this specification. For the reverse connection, the metal wiring line indicated by the dashed line 20 is formed in place of the metal wiring line 19. Further, in the case of the reverse connection, the lower electrode 16 of the DC cut capacitor element is connected to the input terminal 3 (FIG. 1(a)) or the output terminal 4 (FIG. 1(a)) via a metal wiring line (not shown).
The frequency of a voltage-controlled crystal oscillator must be tunable within a user requested range, while ensuring output signal stability. The user requested frequency tuning range varies depending on the application of the oscillator. It is therefore desirable that the frequency tuning range be made as wide as possible so that the same voltage-controlled crystal oscillator can be used in a wide variety of applications.
The frequency tuning range has a strong positive correlation with the capacitance variable range of the load capacitor. Therefore, making the capacitance variable range of the load capacitor as wide as possible is the most important requirement for improving the frequency tuning range. In view of this, attention must be paid to the relationship between the DC cut capacitor element and the variable-capacitance element. The load capacitance value of the voltage-controlled crystal oscillator is the sum of the capacitances of the DC cut capacitor element and variable-capacitance element connected in series. Therefore, if maximum use is to be made of the variation in the capacitance of the variable-capacitance element to vary the load capacitance, it is desirable that the fixed capacitance value of the DC cut capacitor element be made as large as possible.
Before crystal oscillator circuits of integrated circuit form were commercially implemented, discrete component elements were used for the variable-capacitance element and DC cut capacitor that provide the load capacitance. The value of each individual element could be chosen freely and independently of the others. Based on this concept, the capacitance value Ccut of the DC cut capacitor was chosen to be sufficiently larger, usually more than 10 times larger, than the maximum capacitance value Cvmax of the variable-capacitance element. Even after crystal oscillator circuits of integrated circuit form were introduced in recent years, the above concept has been followed without questioning its validity, and the Ccut/Cvmax ratio of more than 10 times larger has been taken for granted and used in circuit design. However, the inventor questioned the viability of this traditional concept and sought room for an improvement in the crystal oscillators.
An object of the present invention is to present, in a crystal oscillator constructed using an integrated crystal oscillator circuit, circuit design conditions for the oscillator circuit that can improve the oscillator""s frequency tuning range with respect to the capacitance variable range of the variable-capacitance element, compared with that achieved with the prior art. More specifically, an object of the invention is to define the range of Ccut/Cvmax values that can improve the oscillator frequency tuning range and to present effective circuit design conditions for improving the oscillator frequency tuning range.
To attain the above object, the voltage-controlled crystal oscillator of the present invention has the following feature.
(1) A voltage-controlled crystal oscillator comprising a crystal, an amplifier, and a load capacitor, wherein the load capacitor includes a voltage-controlled variable-capacitance element integrated on a semiconductor substrate and a DC cut capacitor element connected in series with the voltage-controlled variable-capacitance element, and the DC cut capacitor element has a capacitance value (Ccut) whose ratio to a maximum capacitance value (Cvmax) of the voltage-controlled variable-capacitance element (Ccut/Cvmax) is more than 0.5 and not more than 10.
Further, it is preferable that the voltage-controlled crystal oscillator of the present invention has at least one.of the following features.
(2) The ratio (Ccut/Cvmax) is more than 0.5 and not more than 4.0.
(3) The ratio (Ccut/Cvmax) is more than 0.7 and not more than 1.8
(4) The maximum capacitance value (Cvmax) of the voltage-controlled variable-capacitance element is more than 15 pF and not more than 50 pF.